Regulatory T (Treg) cells play a critical role in the prevention of cutaneous autoimmune and inflammatory diseases, such as vitiligo, psoriasis, atopic dermatitis and systemic lupus erythematosus. T cell receptor stimulation is a major factor which determines Treg cell differentiation. Therefore, manipulations of antigen processing pathways which diminish T cell receptor ligands may be used to enhance Treg cell development and lessen autoimmune disease. GILT (gamma-interferon-inducible lysosomal thiol reductase) is a prime candidate for such manipulation, because loss of GILT diminishes the presentation of a subset of antigens on MHC class I and II. The long term objective of this project is to determine how antigen processing pathways regulate the balance between T cell-mediated immunity and self-tolerance in the skin. GILT is an enzyme in the lysosomal compartment of antigen presenting cells that is critical for efficient processing of disulfide bond- containing antigens. Our prior studies have shown that GILT is required for MHC class II-restricted presentation of tyrosinase-related protein 1 (TRP1), a clinically-relevant autoantigen in vitiligo and melanoma. We have developed a TRP1-specific T cell receptor transgenic mouse model in which skin-specific Treg cells develop and suppress vitiligo. We hypothesize that the loss of GILT increases peripheral induction of Treg cells that home to the skin and control cutaneous autoimmune responses. In the current proposal we will employ a unique set of genetic and immunological tools to determine the etiology of increased Treg cells in the absence of GILT by investigating whether there is increased intrathymic Treg cell development, peripheral induction of Treg cells, or proliferation of Treg cells in the periphery. We will identify the factors that lad to increased Treg cells in the absence of GILT. We will evaluate the role of GILT in Treg cell differentiation of polyclonal T cells and prevention of cutaneous autoimmunity to demonstrate the clinical significance and broad applicability of altered antigen presentation in Treg cell development. We will determine the location where TRP1-specific T cells are required to prevent vitiligo. The impact of these studies is to increase our knowledge of factors that regulate the development and function of skin-specific Treg cells and to determine how alterations in antigen processing affect the development and function of autoreactive Treg cells. In turn, this knowledge has the potential to yield novel targets (such as GILT) to promote Treg cell differentiation and control autoimmunity. Since the effect of GILT is not limited to the presentation of TRP1 or cutaneous autoantigens, modulation of GILT expression or activity may be a broadly applied therapeutic approach for autoimmune disease.